Surface Quality Inspection for Small Size Optical Components

For general small-diameter optical elements (clear aperture ≤ 100mm), most of them are measured by Fizeau interferometer in the workshop. The principle is shown in Figure D.6 (a). The laser beam is focused at the center of the diaphragm after through the beam expander. The spherical wave emitted by the aperture point passes through the beam splitter and the collimating objective lens in order to form collimated light to the standard lens group. The reflected light from the standard surface of the standard mirror group returns in the original path to form a reference light wave Σ. The other light wave reaches the measured plane through the interferometer and returns, forming an interference fringe of equal thickness with the reference light wave. Since the light path is perpendicular to the standard surface, the optical path difference between the corresponding two stripes is λ, and the corresponding air gap thickness is λ / 2. When the thickness of the air gap is reduced, the interference fringes of the same order will move in the direction of the original optical path difference, thereby determining the surface shape deviation. Specific identification method can refer to the national standard GB / T 2831-2009. The spherical measurement method is similar to the plane measurement, except that the standard mirror group of the interferometer is replaced with a spherical standard mirror group.

1: laser; 2: beam expander; 3: iris; 4: beam splitter; 5: collimating objective lens; 6: flat plane crystal; 7: standard surface; 8: measured surface; 9: imaging lens; 10: Fixed lens; 11: standard lens group; 12 spherical lens under test

It usually use the interferometer to inspect in the laboratory. There are many types of interferometers. In China, optical component companies mainly use ZYGO interferometers (see Figure D.7) for measurement. The diameter of the interferometer plane light wave can reach φ600mm, and its measurement accuracy can reach λ / 40. However, in the actual measurement process, the measurement accuracy is also limited by the standard lenses.

Concave spherical (aspheric) lens and flat lens can be directly replaced with standard lenses for measurement. For convex lens, auxiliary convex mirrors are used for measurement, as shown in Figure D.8.

In recent years, the processing technology of large size high-precision optical components has become increasingly mature, and then with they also have relatively large diameters. When measuring the surface quality, because the standard lens equipped with the interferometer cannot meet the measurement requirements, a compensation lens needs to be made specifically for this purpose. The principle of the inspection method is shown in Figure D.9. The optical elements measured with compensating lens have relatively large sizes. Because the environmental impact is relatively large during the measurement, the original data of the interferometer can be extracted and the average value of multiple measurements can be used as the final measurement result.

1: interferometer; 2: standard lenses group; 3: measured surface; 4: standard lens surface

1: interferometer; 2: standard lens group; 3: compensation lens group; 4: measured lens surface

Since large optical component materials are expensive, each process must be strictly controlled. Large-size optical lenses are mainly processed in three processes: rough grinding, fine grinding and polishing. It needs geometric measurement during rough and fine grinding. Mainly measure the shape of the working surface quality during the polishing process. For rough grinding and fine grinding, three-coordinate detection is currently used, as shown in Figure D.10. For polishing, it is mainly detected by interferometer, as shown in Figure D.7 and Figure D.9.

During the polishing inspection of large optical components, a knife-edge instrument can be used for inspection. For experienced processing personnel, detection with a knife-edge instrument can intuitively reflect the shape of the surface, as shown in Figure D.11. There are existing digital knife-edge instruments. The processing personnel can perform local polishing according to the interference pattern displayed by the knife-edge instrument to modify the surface quality.